I think the button's background and borders are the cause of your headache.
Try something like this:
.custombtn { margin: 10px; width:163px; height:43px; background-image:url('images/normal.png'); background-color: transparent; border: none;}.custombtn:hover { background-image:url('images/hover.png');}.custombtn:active { background-image:url('images/click.png');}